In the Bayer process, a concentrated sodium aluminate solution is produced by digesting milled bauxite in a caustic solution under conditions of elevated temperature and pressure. During the milling and digestion steps, a variety of species other than alumina are also extracted and enter the liquor stream. In the Bayer process, these species are generally considered to be undesirable impurities, and due to the cyclic nature of the Bayer process, can accumulate in the refinery's liquor streams unless they are removed by some means. Each of these species will eventually reach a steady state concentration that is a function of the input with bauxite and other sources and the output with the refinery's red mud residue, side-stream removal processes and with the product alumina.
Efficient removal processes for many of the most prevalent Bayer process impurities have been developed. For example, a process for the removal of carbonate ions is described in commonly owned International Application No. PCT/AU99/00757. Sulphate and/or oxalate are removed from caustic aluminate solutions in a process disclosed in commonly owned International Application No. PCT/AU00/00208. However, there are many other species that are typically present at low or trace concentrations in the liquor stream for which to date there are no specific processes for removal or control. These include fluoride and a number of metallic oxo-anions, such as vanadate.
Often, the concentration of most of these species is low because their content in bauxite is low, and also because they are partially removed in an uncontrolled manner as a side effect of the main unit operations of the Bayer process. One of the most important and undesirable of these side-reactions is incorporation of these anions into gibbsite during precipitation, which results in contamination of the refinery's product.
More harmful effects can occur if one or more of these impurities starts to accumulate. The fluoride anion is particularly damaging, as it is capable of forming low-solubility sodium double salts with a number of other anions, such as sulphate, phosphate, silicate and vanadate. This will tend to occur in the most concentrated liquor streams in the alumina refinery, particularly the evaporators, forming a scale that reduces the evaporator's efficiency, adversely affecting productivity and increasing operating costs. At sufficiently high concentrations, these double salts can also co-crystallise in the precipitation circuit, resulting in poor physical and chemical product quality characteristics and very likely provoking a severe loss of productivity.
It is common in some alumina refineries to add lime prior to, or within, digestion to control the phosphate and titanate concentration in the liquor stream, ostensibly to precipitate these anions as calcium phosphate or calcium titanate. This procedure can be effective in controlling these impurities, but is inefficient and can lead to increased calcium contamination in the refinery's product.
To the inventors' knowledge, there have been no processes published which deal with the removal of sodium fluoride or sodium vanadate from caustic aluminate solutions such as Bayer refinery liquors. However, several processes describe the recovery of the aluminium and fluoride values from spent potlining material from aluminium reduction cells. In U.S. Pat. No. 5,470,559, the potlining material is first dissolved in a caustic soda solution to generate a fluoride-rich sodium aluminate solution. This solution is then evaporated to remove fluoride via the crystallisation of fluoride salts. The precipitated fluoride salts are separated from the liquor and then reacted with a calcium hydroxide slurry to precipitate calcium fluoride, while the clarified caustic aluminate solution may be directed to the liquor stream of an alumina refinery. U.S. Pat. No. 5,776,426 discloses a similar process in which the potlining material is simultaneously leached and reacted with lime to precipitate calcium fluoride. The solids are separated, and the clarified liquor again used for some suitable purpose such as Bayer precipitation. In both cases, the caustic aluminate solution generated in the process contains a substantial residual concentration of sodium fluoride, and is hence probably unsuitable for adaptation for use in an alumina refinery. Furthermore, unless the solid calcium fluoride produced in these processes is disposed of in a dry disposal area, it may react with dilute caustic aluminate solutions, releasing the fluoride ions into solution such that they ultimately return to the refinery.
Some of the inorganic anions extracted from bauxite are of potential value if they can be economically purified and recovered. However, the inventors are unaware of any published processes for doing so, other than the solvent-extraction of gallium from Bayer liquors as disclosed in, for example, EP 206081. A process which describes the extraction of rare earth elements from red mud by digesting the mud in a dilute acid solution is disclosed in U.S. Pat. No. 5,030,424, but suffers from the same limitations as all such red mud-based processes, in that the volume of material to be processed is uneconomically large.
Under the appropriate conditions, caustic aluminate solutions will react with calcium from some suitable source such as slaked lime to form the thermodynamically stable and sparingly soluble tricalcium aluminate, Ca3[Al(OH)6]2. This material is commonly referred to in alumina industry parlance as TCA, or C3AH6 in cement industry notation. This reaction is utilised most commonly in the alumina industry to produce TCA crystals of a controlled particle size for use as a filter aid in the polishing or security filtration facility of the refinery, in which fine residual mud particles are “polished” from the green (or pregnant) liquor stream. The use of TCA for this purpose, and a process for the creation of an improved TCA filter aid are described in commonly owned International Application No: PCT/AU01/00886, filed on 20 Jul. 2001.
It has been known for some time that if the sodium aluminate solution also contains an appreciable concentration of sodium silicate, then at least some of the silicate will become incorporated into the tricalcium aluminate. This has been interpreted as a solid solution of calcium aluminosilicate within tricalcium aluminate, with the silica replacing water (Roach, Light Metals 2000, 97, Ed. R. D. Peterson, The Minerals Metals and Materials Society, 2000). To the inventors' knowledge, this behaviour has not been specifically utilised on an industrial scale to remove sodium silicate from Bayer process liquors.
It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country.
From the preceding discussion, while there are a number of processes available which remove impurities that are at relatively high concentrations in caustic aluminate solutions, there remains a need for a process which can remove some of the other impurity anions present at low or trace concentrations, in a form that allows easy and environmentally acceptable disposal or recovery.